Steganography refers to a process of hiding information in other information. One form of steganography is digital watermarking. Digital watermarking is a process for modifying media content to embed a machine-readable code into the data content. The data may be modified such that the embedded code is imperceptible or nearly imperceptible to the user, yet may be detected through an automated detection process. Most commonly, digital watermarking is applied to media such as images, audio signals, and video signals. However, it may also be applied to other types of data, including documents (e.g., through line, word or character shifting), software, multi-dimensional graphics models, and surface textures of objects.
Digital watermarking systems have two primary components: an embedding component that embeds the watermark in the media content, and a reading component that detects and reads the embedded watermark. The embedding component embeds a watermark by altering data samples of the media content in the spatial, temporal or some other transform domain (e.g., Fourier, Discrete Cosine, Wavelet Transform domains). The reading component analyzes target content to detect whether a watermark is present. In applications where the watermark encodes information (e.g., a message), the reader extracts this information from the detected watermark.
The present assignee's work in steganography, data hiding and watermarking is reflected in U.S. Pat. No. 5,862,260; in application Ser. Nos. 09/503,881 (now U.S. Pat. No. 6,614,914) and Ser. No. 09/452,023 (now U.S. Pat. No. 6,408,082); and in published specifications WO 9953428 and WO0007356 (corresponding to U.S. Ser. No. 09/074,034, now U.S. Pat. No. 6,449,377, and Ser. No. 09/127,502, now U.S. Pat. No. 6,345,104). One application of watermarks, sometimes referred to as fragile or semi-fragile watermarking, is to detect alteration of media signal. Some of assignee's work in this area is reflected in co-pending application Ser. No. 09/234,780, filed Jan. 2, 1999, Ser. No. 09/498,223, filed Feb. 3, 2000 (now U.S. Pat. No. 6,574,350); 60/198,138, filed Apr. 17, 2000, Ser. No. 09/574,726, filed May 18, 2000, Ser. No. 09/618,948, filed Jul. 19, 2000 (now U.S. Pat. No. 6,385,329), Ser. No. 09/625,577, filed Jul. 25, 2000, and Ser. No. 09/645,779, filed Aug. 24, 2000. A great many other approaches are familiar to those skilled in the art. The artisan is presumed to be familiar with the full range of literature about steganography, data hiding and watermarking.
The invention provides methods and systems for embedding and decoding watermarks in frequency domain regions of media signals, such as image, audio and video signals. One aspect of the invention is a method for embedding auxiliary information in a media signal such that the auxiliary information is substantially imperceptible. The method transforms the media signal into frequency transform regions, calculates a hash of the media signal in one frequency transform region, and embeds the hash into a watermark in another frequency transform region. In a compatible watermark decoder, the hash is extracted and compared with a recalculated hash to detect signal alteration, and identify the location and type of alteration.
Another aspect of the invention is a method of decoding a message signal that has been embedded in a media signal such that the message signal is substantially imperceptible in a perceptual domain of the media signal. The method transforms the media signal from a perceptual domain to frequency domain regions, where each region includes a set of frequency coefficients within a frequency range. It then decodes a watermark message from at least one of the regions, and compares the watermark message with a reference derived from another frequency domain region to determine alteration of the media signal. The reference may be a hash of the frequency domain region from which it is derived, another instance of the watermark message, or some other reference signal.
Another aspect of the invention is a watermark decoder. The decoder comprises a frequency domain transform operator for transforming the media signal from a perceptual domain to frequency domain regions, where each region includes a set of coefficients within a frequency range. It also has a message decoder for decoding a message signal having two or more elements from at least one of the frequency domain regions. The decoder is operable to compare the message signal with a reference signal derived from a frequency domain region different than the region or regions from which the message signal is decoded to detect alteration of the media signal.
This document describes a variety of implementation details and applications of watermark embedders and decoders. In addition to the message signal, for example, the embedder may also encode an orientation signal to synchronize the decoder with the embedded signal in a distorted version of the watermarked signal.
The watermark system may be used in a variety of applications, including robustly carrying metadata or links to metadata, and to detect alterations of the watermarked signal, such as alterations due to printing, scanning, compression, etc. The watermark may be used to detect alteration by evaluating changes to instances of a message signal decoded from a host media signal. In one application for example, a watermark decoder evaluates changes in a watermark message signal relative to a reference message signal. The reference may be the original message signal embedded in the host media signal, instances of the message signal decoded from different parts of the media signal (e.g., different subbands), or a combination of both. In addition, the watermark may be used to carry authentication information, such as a hash of the host signal, that is used to detect alteration. The watermark decoder compares the hash extracted from the watermark with a hash re-computed from the host signal to detect alteration. The decoder may also use the extent and nature of the detected alteration to distinguish among different types of alterations, such as those due to printing, scanning, compression, etc.
Further features will become apparent with reference to the following detailed description and accompanying drawings.